Conventional printed circuit boards are formed by utilizing cloth, particularly of fiberglass, which is then impregnated with a resin such as epoxy, and partially cured to a "B" stage condition. Copper foil treated on one side to effect good bonding may then be placed on one or two sides of the pre-impregnated "B" stage material. The thusly resulting assembly is placed between steel platens with a suitable mold release or a release film between the platens and the assembly is cured under heat and pressure to form a laminate, copper clad on one or two sides.
The copper clad or possibly unclad laminates are subsequently used for the production of printed circuit boards by a subtractive or additive process. Printed circuit boards are used in the electronics industry for circuitry. In general, holes have to be formed in the structure for the insertion of component leads (i.e., for resistors, capacitors, integrated circuit chips, transistors, etc.) or for the interconnection of circuits from one surface to one or more other surfaces.
The forming of holes (usually by drilling) in laminates reinforced with fiberglass or other hard reinforcing fiber material is difficult, expensive and creates problems which necessitate additional operations. The drilling runs into problems such as epoxy smear where the drill heats up, due to friction, to a temperature higher than the glass transition temperature of the plastic matrix and causes this plastic to smear over the edges of the copper. This smear must be removed by etch back to re-expose the copper to insure a proper connection with the thru-hole plated copper.
In researching prior art patents for purposes of examining the uniqueness of the present invention, U.S. Pat. No. 3,537,937 dated Nov. 3, 1970 has been encountered. Therein is disclosed an arrangement in accordance with which longitudinally arranged filaments are applied to opposite surfaces of an endless flat metallic band and additional filamentary rovings are helically wound thereover. The filamentary structure is resin impregnated before the band and structure pass through a resin curing apparatus wherein the resin is partially cured. Upon emerging from the curing apparatus, the resin and filamentary material along the edges of the band is removed and separate top and bottom filament would resin sheets of continous length are removed from the band and wound on spools.
This prior art could be suited for producing structural panels for mechanical loads. It does not contain teaching on structurally stable and balanced and flat thin sheets.
The method of U.S. Pat. No. 3,537,937 cannot produce layers wherein the filament sections are perfectly perpendicular to one another.
It is, accordingly, an object of the present invention to provide an improved laminate adapted for use in the manufacture of printed circuits and an improved method for making the same.
It is a further object of this invention to provide an improved laminate adapted to permit precise positioning of components.
It is yet another object of this invention to provide a filament-wound resin-bonded structure capable of being punched or at least drilled without the drills touching the reinforcing fibers. Additional objects of the invention are set forth below.
It is further object of the invention to provide for the design and control of the thickness of laminates, and the reinforcement-content-to-resin-content ration to values and tolerances not attainable with the previously known state of the art. The number of variables in the previously known process (such as weight of fabric, resin content, age and rheology of the prepreg, variation in heat history in all directions X, Y and Z of the book, i.e.; many layers of laminates and separator plates between platens in a press lead to wide variations in temperature versus time between laminates and even in parts of the same laminates. In the present invention, the laminate is molded to the exact thickness required with precise repeatability. The reinforcement can moreover be applied in situ in the exact amount the design calls for, even compensating for its yield variation, i.e., yards per pound. The resin matrix takes up the rest of the space and there are no voids. All this leads to latitude of design and tight control of wall thickness and reinforcement-to-resin ratio.
To achieve the above and other objects of the invention there is provided in accordance with a preferred embodiment of the invention a method which comprises forming a set of electrically non-conductive filament sections in at least substantially mirror image relationship about a plane of symmetry.
Viewed from another aspect, a reinforcement structure is configured in planar layers of at least substantially equally spaced parallel filaments. The direction of the filament sections may alternate in alternate layers and, in a preferred contruction, alternate layers are perpendicular to each other. It is also preferred to have the number of filament sections in the two perpendicular directions equal. It is further preferred that the filaments form a mirror image about the neutral plane or the center of the laminate or neutral axis thereof. According to the above preferred version of the invention, the angle between filament sections in different layers is substantially perpendicular such that the filament sections in adjacent layers are orthogonally related to one another. However, other angles can be used for special cases, and two or more parallel layers can be put together.
As will be shown hereinbelow the filament sections of each layer are a number of single or multi-filament arrangements would helically (or the like) and continuously around a flat form. Alternatively the filaments can be wound on some other suitable form and transferred to a flat form either layer by layer or in groups of layers. This filament arrangement may preferably be a bundle of untwisted filaments of the type commercially available in relatively flat form such as strand or roving, and ideally suitable for employment in accordance with the method of the invention.
According to one aspect of the invention, the filament arrangement is wound helically around a flat form which is preferably made in the shape of a right quadrilateral. Alternate layers or two or more parallel layers are respectively wound in criss-cross directions on this form. According to yet another aspect of the invention, the filament sections are embodied in a matrix of electrically non-conductive material which fixes the filament sections in position, and fills all the spaces or interstices between the two outer surfaces of the board not filled with filaments.
The layers are prefereably arranged in parallel planes. Moreover, in accordance with a preferred aspect of the invention, the aforesaid form has opposite sides connected by edges and the sets are formed on opposite sides of the form, the filament arrangement being brought from one side to the other of the form by traversing the edges thereof. The method further includes separating the sets by trimming the helically wound filament arrangement at the edges of the form.
According to yet another aspect of the invention, each filament arrangement employed is wetted during the winding on the form with a resin which is subsequently cured to form the aforesaid matrix. As an alternative, the filament arrangements may be wetted subsequent to the winding of the form by being immersed in a bath of resin or the like. The preferred method is to apply the resin in a vacuum impregnation method. An alternative is to partially impregnate prior to or during winding followed by vacuum impregnation with the same or compatible resin systems. Another alternative is to impregnate prior to winding.
As has been indicated hereinabove, the filament arrangements are formed as bundles of filaments. These bundles may be helically wound by effecting a rotation of the form. As an alternative, the filament arrangements may be helically wound on the form by revolution of at least one planetary source of filament arrangements about the aforenoted form.
In accordance with yet another aspect of the invention, an angle is controlled as between the form and the filament arrangements by relatively displacing the form and at least one source of the filament arrangement or arrangements. With respect to and in further accordance with the invention, the method may comprise forming a conductive surface on at least one of the layers formed thereby. Another aspect of the invention involves the feature of cutting a laminate out of each of the above-indicated sets. The laminate may be cut in quadrilateral form to have side edges generally perpendicular to the filament sections.
In accordance with still further aspects of the invention, the filament sections as will be seen may be of a material such as, for example, fiberglass, quartz, aramid or the like. The matrix may be of plastic resin and may be, for example, epoxy, polyimide, bismaleimide, polyester, vinyl ester, or polybutadiene. In the above-indicated arrangement, the layers are preferably planar and the filament arrangement is maintained under controlled tension as it is wound around the form. This provides for pre-stressing the filament sections and provides for a control of the various physical characteristics of the resulting product or products.
Yet another feature of the invention relates to bordering the above-noted form with a disposable rim which may be plastic and which is removable when the filament arrangement is trimmed as aforesaid. In accordance with the invention, the pitch of the helically wound arrangement may be controlled by controllably displacing the aforenoted form.
It is further within the purview of the invention to arrange an equal number of filaments in the X as in the Y direction, this constituting an especially advantageous feature of the invention which is useful in the sensitive applications which are envisaged. This is to get an equal T.C.E. (Thermal Coefficient of Expansion) in the X as in the Y direction. Moreover, the product can be arranged to have an essentially equal modulus of elasticity in both of the X and Y directions or a tailored different modulus in the X as in the Y direction.
While achieving the aforenoted advantages as well as objects of the invention, the filament sections may be arranged to form a spacing to accomodate a passageway through the resulting product in the Z direction which is perpendicular to the plane of X and Y. Other voids are, however, avoided by making the matrix of a curable substance which is cured with the filament sections therein and with the curing being effected with heat, and vacuum being employed prior to curing to remove all air and voids from the interstices, and accurate spacers can be employed to determine the final thickness of the product.
In the process of the present invention, a predetermined set of holes, which are void of reinforcing filaments, can be formed through the wound reinforcing layers. These holes can then be filled with resin in a molding operation and subsequently drilled or punched. By use of suitable tooling, the holes can also be formed in the molding operation, either in whole or in part, eliminating or reducing the drilling or punching operations required for use of the product.
In the preferred version, where the desired holes are filled with plastic in the absence of any reinforcing fibers, it is then possible to punch the holes depending on the properties of the plastic matrix. In the event that drilling is necessary, the drilling speed will be faster and will generate less heat, the drills or bits will last through the drilling of many more holes, the holes will be cleaner and can also be smaller, and there will be no resin smear, thus eliminating the need for etch back and accompanying problems.
Where the holes are formed, punched or drilled through the resin only, the margins will be resin only. This solid resin annulus surrounding the hole will act as a barrier to prevent any of the etching, plating, rinsing, or other such solutions from penetrating into the reinforcing fibers, the resin/reinforcement interface and micro-voids therein, or the interlaminar area. The solutions will also be prevented from penetrating any seeds, voids or hollows that may be present in the reinforcing fibers. In the current state of the art, all of the above areas are open to be penetrated and contaminated by the various solutions or residues therefrom. These solutions and residues can be conductive in which case, they can form unwanted conductive paths or short circuits. They can also be corrosive and erode the conductor and/or dielectric and cause failure in the circuit over a period of time.
In accordance with yet another feature of the invention, the above-noted conductive surface is formed by placing a metal foil on at least one side of the form or outer molding plate, the foil being transferred to the adjacent surface of the product. The alternative to the use of a metal foil in accordance with the invention involves plating at least one side of the form or outer molding plate with a metal which is capable of being transferred to the adjacent surface. It should be noted that it is within the scope of the invention to contemplate placing the conductive surface on the product in the configuration of a circuit on at least one side of the form or outer molding plate and transferring the metal as aforesaid. The conductive surface may be burnished or ground to achieve desired granularity and/or thickness.
The above objects, features and advantages of the invention relate to the method. However, it is to be noted that the invention also contemplates one or more structural configurations such as, for example, a structure which comprises a matrix and, embedded in said matrix, a plurality of filament sections arranged in parallel layers as implied by the method set forth in generalities hereinabove. The filament sections in each respective layer are preferably parallel and the filament sections in alternate layers may be arranged angularly with respect to one another. The angle is prefereably such that the filament sections in the alternate layers are at least substantially perpendicularly related. Advantageously for the end results contemplated within the scope of the invention, the filament sections and matrix may be electrically non-conductive. The layers may, moreover, as implied hereinabove, be of right quadrilateral configuration having pairs of opposite edges, the filament sections being at least substantially perpendicular to one of the pairs of edges. As also implied above, the filament section may be prestressed.
It follows from the method of the invention that the structure provided in accordance with the invention may include a metallic coating and the matrix may have at least one surface on which at least part of the metal coating is supported in accordance with one emodiment of the invention.
It also follows from the method of the invention that, if the matrix is provide with at least one hole, it will define an interior surface within the matrix. In this case the metallic coating may include a portion supported on at least a part of the aforesaid surface.
In the aforesaid structure, the layers are such that all of them can have a substantially equal number of filament sections. Alternatively, the layers can have a different number of filament sections by design in order to tailor the flexural modulus in the X or Y direction. The filament sections in each of the aforsaid layers are uniformly disposed about an axis, the axes of the alternate layers being transverse to one another in accordance with a preferred embodiment of the invention.
As has been noted already, herein, a preferred product of the invention is a printed circuit board. Since this board will be fabricated of the structure noted hereinabove, it will probably comprise a matrix having a plane of symmetry and, on opposite sides of this plane and within the matrix, it will include at least first and second layers of parallel filament sections, the filament sections in the first layer being angularly and preferably perpendicularly related to the filament sections in the second layer. Also, preferably but not exclusively, the parallel filament sections of the first layers on opposite sides of the plane of symmetry are at least substantially parallel to each other and furthermore they are preferably aligned with each other. Also, preferably, and in accordance with the invention, the aforesaid layers will have a like number of filament sections. These filament sections may be, according to one embodiment, under tension or, in other words, are prestressed.
As will be described in greater detail hereinbelow, the layers have respective axes which are transversely related, the filament section in the respective layers being uniformly distributed about the correponding axis. It will be seen that the matrix may have at least one surface comprising a metallic coating on at least part of the surface, thereby enabling the production of a printed circuit board. It has been implied above, and will be described in greater detail hereinbelow, that the filament sections can be arranged in the matrix to leave a portion of the matrix devoid of filament sections whereby the matrix may be provided with a hole extending through this portion. The printed circuit board of the invention will also involve the feature whereby the filament sections in the first layer are perpendicular to the filament sections in the second layer. Also involved is the feature whereby the matrix is or includes a section in the shape of a right quadrilateral with two faces in the shape of parallel planes and having four edges arranged in parallel pairs connecting these faces. In this arrangement, the filament sections of the first and second layers will be at least substantially perpedicular to respective of the pairs of edges.
While preferably the filament sections of the invention will be of untwisted filament structure, they may also be of twisted yarn or plied yarn structure. The filament sections may preferably be of fiberglass, aramid, quartz, carbon, nylon, polyester or the like. As set forth hereinabove, the matrix is preferably of a material such as bismaleimide, epoxy, polyimide, polyester, vinyl ester, phenolic, melamine, polybutadiene or the like.
The invention also involves the aspect of providing an apparatus for the preparation of a filament reinforced matrix. This apparatus will comprise generally a form, a source of filament, and a drive arrangement to effect a relative motion between the source and form such that the filament is wound about the form in at least first and second helices having orthogonally related axes. The apparatus of the invention may also comprise an arrangement for applying a settable resin to the helices to constitute the matrix. As noted above, a form is employed. This form may preferably be a flat mandrel or frame. The drive arrangement may include an arrangement to rotate this mandrel and for rectilinearly displacing the mandrel with respect to the source while the mandrel is rotating. Alternatively, the drive arrangement may include elements to displace the source in encircling relationship to the mandrel so that the source revolves about the mandrel. In this case, the drive arrangement will also include elements to displace the mandrel rectilinearly while the source is encircling the same.
In further accordance with the invention, there may be provided an arrangement to hold the mandrel sequentially in perpendiculary related postures for respective of the helices. A specific feature of the invention will provide that the mandrel include removable coating of conductive metal which is adapted for transferred to the aforesaid matrix.
Yet another feature of the invention involves that the mandrel be a flat metal plate having edges, there being furthermore provided removable plastic or other disposable rim members attached to these edges and adapted for being transversed by the aforesaid helices and being moreover adapted to facilitate a trimming of the helices at these edges.
Yet another feature of the invention relates to the provision of an arrangement for applying the settable resin in the form of a chamber to receive the form with the helices thereon, there being a source of resin operatively associated with the chamber and a further arrangement to effect a vacuum within the chamber to enhance penetration of the resin into the helices and render the same devoid of bubbles and other such voids. A further particular significant feature in the molding process is the use of outer plates with means to bring them into contact with stops and impregnated helices while under vacuum.
Still another feature of the invention involves the provision of a cam or electronic arrangement operatively associated with the source and/or the form with the filament to provide speed compensation and tension control for the supply of filament during the winding of the helices.
The above and other objects and advantages of this invention will be apparent from the detailed description which follows, as illustrated in the accompanying drawing.